Generally, automatically shiftable vehicle transmissions of planetary design are already described in the state of the art many times over, and are subject to continuous development and improvement. Such transmissions should feature a sufficient number of forward gears and one reverse gear and a transmission ratio that is very well-suited for motor vehicles with a high overall spread, favorable progressive steps and a sufficiently large start-up transmission ratio for the specific application. In addition, this transmission should require low construction costs, in particular a low number of shift elements and, upon a sequential shifting operation, should avoid so-called “group gearshifts,” such that, when there are gearshifts to the next higher or next lower gear, only one previously engaged shift element is open, and one previously open shift element is engaged.
Such a multi-speed automatic transmission is described, for example, in DE 10 2005 002 337 A1 of the applicant. It essentially includes one drive shaft and one output shaft, a total of eight rotatable shafts, a total of four individual planetary gear sets and five shift elements. Through the selective engaging of three of five shift elements designed as clutches and brakes, a total of eight forward gears can be shifted without any group gearshifts.
Alternative 8-speed automatic transmissions are described in DE 10 2009 047 271 A1 of the applicant, each of which comprises four individual planetary gear sets, five shift elements and a total of nine shafts. In a first variant, a transmission is proposed, in which, in contrast to DE 10 2005 002 337 A1, the ring gear of the first planetary gear arranged close to the drive, the sun gear of which is constantly connected to the sun gear of the second planetary gear set connected to the drive shaft and can be fixed through the first shift element at the transmission housing, is firmly connected to the transmission housing. Thereby, the second shift element is arranged in the power flow between the planetary gear carrier of such first planetary gear set and the ring gear of the fourth planetary gear set connected to the output shaft. In a second variant, a transmission is proposed, in which, in contrast to DE 10 2005 002 337 A1, the ring gear of the first planetary gear arranged close to the drive is firmly connected to the transmission housing, whereas the second shift element is arranged in the power flow between the sun gear of such first planetary gear set and sun gear, which can be fixed on the transmission housing through the first shift element, of the second planetary gear set connected to the drive shaft.
A further development of the 8-gear automatic transmission described in DE 10 2009 001 253 B3 of the applicant. Here, through the addition of a sixth shift element formed as a clutch in the power flow between the drive shaft and the second planetary gear set that can be blocked through the engaging of the fifth of the previously present five shift elements, an additional forward gear can be presented, which, from its transmission ratio, is larger than the previous first gear of the 8-gear transmission, thus an additional start-up gear below the previous first gear of the 8-gear transmission.